In order to further improve the fuel efficiency of hydrocarbon fuel engines there is interest in operating the engine in a fuel-lean combustion mode. For gasoline engines this means introducing an air/fuel mixture at a ratio of about seventeen to twenty three parts by weight of air per part of gasoline. For diesel engines the air to fuel mass ratio is even higher. The purpose of fuel lean operation is to obtain more complete combustion of the fuel.
Attractive as the lean-burn engines have become lately for their superior fuel efficiency, there remains a major technical barrier to the automotive application of lean-burn engine technology. It is associated with NOx emission in the engine exhaust. The exhaust gas from a lean-burn gasoline engine is typically at a temperature of 300° to 600° C. during warmed up engine operation. And the exhaust contains water, small amounts of carbon monoxide and unburned hydrocarbons (e.g., ethylene), nitrogen, and nitrogen oxides (NO and NO2). The challenge is to promote the reduction of NOx in this chemically oxidizing environment.
The traditional three-way catalysts while active for NOx reduction under stoichiometric exhaust conditions, are not effective in reducing NOx under highly oxidizing conditions prevailing in the lean-burn engine exhaust. Lean-NOx reduction technologies currently available are not sufficiently effective to meet future stringent emission standards either. This has prompted intensive and extensive R&D activities around the world for improved lean-NOx reduction technology.
Among a few different approaches for lean-NOx reduction, the selective catalytic reduction of NOx using unburned hydrocarbons (HC-SCR) as reductants has been attracting the most attention. There are quite a few known lean-NOx reduction catalysts for the HC-SCR process. Among those reported in the literature, Cu/ZSM-5 zeolite is probably the most studied catalyst for high temperature applications, whereas Pt/ZSM-5 is for low temperature applications. In most lean-NOx catalysts, zeolites are used as catalyst support on which the active metals are ion exchanged. Among many different zeolites, the ZSM-5 zeolites with high silica content have been preferentially used for lean-NOx catalysts. Unfortunately, however, all those catalysts suffer from the combination of the narrow effective operating temperature window and insufficient catalytic activity and hydrothermal stability.
All zeolite-based catalysts, Cu/ZSM-5 in particular, have major problems due both to hydrothermal degradation and negative sensitivity towards water vapor and SO2. In general, the permanent loss of activity has been attributed by investigators to (a) degradation of the support, (b) irreversible loss of Cu2+ from the zeolite framework or (c) combination of the above. The Cu1+ is known to be the active catalytic site for both NO decomposition and NO reduction with hydrocarbon. The inter-conversion between Cu1+ and Cu2+ depends on the reaction conditions including temperature and the types of reductant. Hydrothermal de-alumination of the zeolite framework has been a major issue in the deactivation of the catalyst. It appears that deactivation is mainly caused by migration of Cu2+ ions to locations inside ZSM-5 where their reduction to Cu1+ is more difficult. The above mentioned studies clearly reveal that Cu/ZSM-5 deactivates substantially even under relatively mild conditions and indicate that a dramatic increase in hydrothermal stability is required for the catalysts if they are to be used in the automotive application. Thus, the search continues for better lean-NOx catalysts, which requires both more stable supports and more active catalytic chemical ingredients.
Accordingly, it is an object of this invention to provide a stable and effective catalyst for reduction of NOx in a lean burn exhaust such as from a hydrocarbon-fueled automotive vehicle engine.